1. Field of the Invention
This invention relates to an optical scanning apparatus and an image forming apparatus using the same, and particularly is suitable for an image forming apparatus such as, for example, a laser beam printer having an electrophotographic process, a digital copying machine or a multifunction printer adapted to record image information by the use of an overfilled optical scanning apparatus.
2. Related Background Art
A so-called underfilled optical scanning apparatus (hereinafter referred to as “UFS”) in which the width of the deflection surfaces (reflecting deflection surfaces) of a rotating polygon mirror (hereinafter referred to as a polygon mirror) along the rotational direction thereof is made greater than the width of a light beam incident on the polygon mirror along a main scanning direction has heretofore been popular as an optical scanning apparatus in which a light beam is deflected in the main scanning direction by a polygon mirror as a light deflector and a surface to be scanned is scanned with the light beam.
On the other hand, in an image forming apparatus such as a laser beam printer or a digital copying machine, the higher speed of image recording and higher resolution are required. Particularly, with the spread of the coloring of image forming apparatuses in recent years, an image output of a high speed and high resolution is required more and more.
In the underfilled optical scanning apparatus as described above, in order to meet the requirement for a higher speed, a method of making the rotational speed of the polygon mirror higher, and a method of increasing the number of the surfaces of the polygon mirror are conceivable.
However, it is not easy to make the rotational speed of the polygon mirror higher because it poses various problems such as the heat generation of a motor for driving the polygon mirror, electric power consumption, windage loss occurring during the rotation of the polygon mirror, and noise. It is also not easy to increase the number of the surfaces of the polygon mirror while maintaining the width of the deflection surface along the rotational direction because it causes the bulkiness of the polygon mirror, and poses various problems similar to those noted above.
Also, higher resolution can be obtained by widening the width of the deflection surfaces of the polygon mirror along the rotational direction and making the F number (F No.) of an imaging optical system (scanning lens system) small. However, this likewise results in the bulkiness of the polygon mirror.
So, in order to prevent the size of the polygon mirror from enlarging and to increase the number of the deflection surfaces, a so-called overfilled optical scanning apparatus (hereinafter referred to also as “OFS”) in which the width of the deflection surfaces of the polygon mirror along the rotational direction is set smaller than the width of the light beam in the main scanning direction has been put into practical use.
The above-described overfilled optical scanning apparatus, however, is an optical scanning apparatus suitable for high speed and high resolution, but because of the increased number of the deflection surfaces of the polygon mirror thereof, as compared with the underfilled optical scanning apparatus, the angle at which a deflection surface of the polygon mirror can scan becomes narrower, it is necessary to make the focal length of the imaging optical system (scanning lens system) long in order to scan the same scanning width. Therefore, the optical path length of the optical scanning apparatus becomes long, thus posing the problem that the entire apparatus becomes bulky.
Proposed has been an optical scanning apparatus which solves the above-noted problem (see Japanese Patent Application Laid-open No. 2000-267030).
In Japanese Patent Application Laid-open No. 2000-267030, the imaging optical system is constituted by two scanning lenses, and the inverse number of the combined focal length of the two scanning lenses in the deflection surfaces is made smaller than the inverse number of the system focal length of the optical scanning apparatus, thereby achieving a compact overfilled optical scanning apparatus in which the distance from the polygon mirror to the surface to be scanned is short.
Generally, making the inverse number of the combined focal length of the imaging optical system in the deflection surfaces smaller than the inverse number of the system focal length of the optical scanning apparatus can be achieved by converging a light beam incident on the imaging optical system in the main scanning direction.
In the above-mentioned Japanese Patent
Application Laid-open No. 2000-267030, however, the light beam incident on the imaging optical system is set to a convergent light beam (convergent beam) relatively strong in the main scanning direction, and the focal length of the imaging optical system in the main scanning direction is set to the negative. Therefore, when an unevenness amount δL exists in the distances from the rotation center of the polygon mirror to a plurality of deflection surfaces, there is the problem that great positional deviation occurs to an imaged spot on the surface to be scanned.
The above-mentioned positional deviation of the imaged spot on the surface to be scanned occurs without fail in principle if the light beam incident on the imaging optical system is a light beam convergent in a main scanning section, and cannot be avoided.
Also, it is a well known fact that the more a convergent light beam converges, the greater the above-mentioned positional deviation of the imaged spot on the surface to be scanned becomes. For example, in the underfilled optical scanning apparatus, Japanese Patent Application Laid-open No. H09-318874 discloses a technique that the optical scanning apparatus can be configured compact by making a light beam incident on the imaging optical system to be a light beam convergent in the main scanning direction, and the positional deviation of the imaged spot on the surface to be scanned can be mitigated by appropriately setting the degree of convergence of the above-mentioned convergent light beam where the unevenness amount δL exists in the distances from the rotation center of the polygon mirror to the plurality of deflection surfaces.
However, again in Japanese Patent Application Laid-open No. H09-318874, if the degree of convergence of the light beam incident on the imaging optical system is intensified, the positional deviation of the imaged spot on the surface to be scanned increases as previously described and therefore, the degree of convergence cannot be intensified and accordingly, there has been a limitation to the shortening of the optical path length of the optical scanning apparatus.